We hope to understand the structure and regulation of expression of the gene for prolactin. cDNA complementary to rat prolactin messenger RNA will be synthesized and cloned in bacteria. Its nucleotide sequence will be determined. The cloned DNA will be used as a probe to purify the prolactin gene from the natural DNA and to understand the hormonal control of prolactin gene expression. The nucleotide sequence of the natural prolactin gene as well as its proximity to other identifiable genes will be determined.